You say that, but it happens — "Experts Exchange", for example, certainly used to try to hide the answers from users who hadn't paid while encouraging search engines to index them.
That's not quite the same. Experts Exchange wanted the content publicly searchable, and explicitly allowed search engines to index it. In this case, many customers probably aren't aware that there is a separate search index that contains much of the data in their private documents that may be searchable and accessible by entities that otherwise shouldn't have access.
